Copper pour or traces?

At 400 Hz, practically anything will work.

Copper pours don't seem to warp boards nowadays, if they ever did.

John

However, providing a little relief around the pads can make them easier to solder and de-solder (ie, put a hole around the pad, and then tie the pad to the pour with 3 or 4 traces, rather than having no pad.) I'll often just run a pattern of round holes in the pour rather than a grid (resulting in square holes), which in thoery is better for etching behavior (less sharp corners) but I'm not sure this is really needed or makes any practical difference. However, it keeps me happy, and does not seem to detract any.

Rule 1: Don't assume anything.

It is unlikely that you will ever find a board with an external power plane. Double sided boards will normally have a copper pour on each side both of which are connected to GND. Having said that, go back and invoke rule 1 above.

Just use solid copper pours. FR4 will not warp if you have copper pours on both sides.

They used to if you didn't balance the thermal mass between both sides from the core layer. In fact, this was the main reason for one of the blitz consulting trips. 35 hours was travel time, the remainder was finding that warpage was the cause of end test failures, donning the old army boots to walk over to a lumber yard (roads too iced up for driving), making a frame, running a few boards.

Provided they are somewhat summetrical to the core. Else go back to rule

We did one board with microwave laminate as the first dielectric, and FR-4 for the rest. It looked like a potato chip. It was cool: you could lay it on your desk and give it a twirl, and it would spin for almost a full minute. It took vise grips to get it out of the card cage.

John